Ordinary moving coil receivers and speakers are referred to as moving coil electrical-acoustic transformation device. No matter if it is applied to a mobile phone or an earphone, rapid drop will occur in the high frequency response curve of the receiver at 6 k-9 kHz and rapid drop will also occur in the high frequency response curve of the speaker after 10 KHz, which is the so-called high frequency cutoff frequency. Due to the limitations of the material and process of the moving coil electrical-acoustic transformation device, it is hard to increase the high frequency cutoff frequency.
With the application of 4G communications, electrical-acoustic transformation devices with super wideband are required. The high frequency cutoff frequency is required to be up to 16 kHz and higher. Thus, the existing moving coil electrical-acoustic transformation device cannot meet these requirements.